This section is intended to provide a background or context. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
Many telecommunications systems require linear radio frequency (RF) power amplifiers. A linear amplifier produces an output signal that is linearly related to the signal applied to the input. An amplifier that compresses its input or has a non-linear input/output relationship may provide degraded intermodulation performance, potentially resulting in interference to other radio channels.
Predistortion is a technique that has been used to improve the linearity of RF power amplifiers. A predistortion circuit inversely models the gain and phase characteristics of the RF power amplifier. When the predistortion circuit is combined with the RF power amplifier, the result is an overall system having enhanced linearity and reduced distortion. In essence, “inverse distortion” is introduced into the input of the RF power amplifier, thereby cancelling any non-linearity the RF power amplifier might otherwise exhibit. Predistortion is a cost-saving technique that enhances power efficiency. RF power amplifiers tend to become more non-linear as their output power increases towards their maximum rated output. Predistortion may be employed to obtain more usable power from the amplifier without having to build a larger, less efficient and more expensive amplifier.
Predistortion may be implemented in the analogue domain as well as the digital domain. For example, some present-day radio transmitters support Digital Pre-Distortion (DPD) for improving intermodulation performance. DPD techniques bring increased power efficiencies to the transmitter, resulting in considerable power savings for users. While it is possible to implement efficiency enhancement techniques within the context of analog devices, DPD algorithms provide additional efficiencies that could not have been achieved solely within analog domain.
DPD is effective at improving intermodulation performance when all inputs to the DPD process are accurate. Some of the key inputs to the Digital Pre-Distortion algorithm are: a) an undistorted transmitted signal; b) a feedback signal; and c) a suitable mathematical model for the radio frequency (RF) power amplifier used by the transmitter. As part of the DPD approach, the undistorted transmitted signal has to be synchronized with the feedback signal. Correct synchronization will match a sample of the transmitted signal to a correct feedback sample. Usually a timing offset has to be computed to define the match from the transmitted signal to the feedback signal. This offset is commonly known as either the timing offset or simply the delay. Throughout this specification, the word “delay” is used to refer to the timing offset between the transmitter and the feedback signal.
In order to achieve enhanced DPD performance, it is important to determine an accurate delay. For most waveforms, this delay has been defined in terms of integer samples of the DPD rate. One example of an integer sample is 1/(307.2 MHz), or 3.25 nanoseconds, where 307.2 Mega-Samples Per Second (MSPS) is the sampling rate or the processing rate. For some waveforms and their occupied bandwidths, determining an integer delay is not adequate.